A III-V group nitride semiconductor has been variously used for an optical device such as blue/green LEDs (light emitting diodes), a high speed switching device such as a MOSFET (metal semiconductor field effect transistor) and a HEMT (hetero junction field effect transistor), light source of an illumination or a display apparatus, and the like. In particular, a light emitting device using an III group nitride semiconductor has a direct transition-type bandgap corresponding to the range of visible rays to ultraviolet rays, and can perform high efficient light emission.
The nitride semiconductor has been mainly utilized as a LED or a LD (laser diode), and research for improving the manufacturing process or light efficiency had been conducted.
GaN is representatively used as the III-V group nitride semiconductor. Such a nitride semiconductor is grown on a substrate through crystal growth. Further, the nitride semiconductor is activated into a p-type semiconductor or an n-type semiconductor according to the type of dopants, thereby forming a P-N junction device or an N-P-N junction device.
The biggest problem in determining the light emission efficiency of the LED is that the external quantum efficiency is low. The external quantum efficiency represents efficiency for emitting light, which is generated from an active layer, to an exterior. A part of the light generated in the active layer is not emitted to an exterior due to refraction index difference in the boundary of a semiconductor layer, and travels in the LED and then attenuated due to total internal reflection. Various methods for improving the external quantum efficiency have been proposed to solve such a problem.